1. Field of the Invention
The present invention relates to an image heating apparatus for effecting heating by eddy currents generated utilizing the electromagnetic induction. More particularly, this apparatus concerns a fixing device in image forming apparatus such as electrophotographic copiers, printers, and facsimile devices, or concerns an apparatus for heating an unfixed toner image formed in direct or indirect fashion on a surface of a recording medium with toner of a heat-melting resin by an appropriate image forming process means such as electrophotography, electrostatic recording, or magnetic recording to fix the toner image as a permanent fixed image on the recording medium surface.
2. Related Background Art
FIG. 12 is a drawing to explain the prior art, which is a schematic sectional view of a laser beam printer as an application of the electrophotography to printer. The operation of this apparatus will be explained.
An electrostatic latent image is formed on a photosensitive drum 11 as modulating the intensity of a laser beam from a scanner 13 according to an image information signal sent from a host computer. The intensity and irradiation spot diameter of the laser beam are properly set according to the resolution of image forming apparatus and the desired image density, and the electrostatic latent image on the photosensitive drum 11 is formed by maintaining portions irradiated by the laser beam at a light potential V.sub.L and the other portions at a dark potential V.sub.D charged by a primary charger 12. The photosensitive drum 11 rotates in the direction of the arrow, so that the electrostatic latent image is successively developed by a developing unit 14. The toner in the developing unit 14 forms a uniform toner layer on a developing sleeve 1402 while the toner height and triboelectric effect are controlled by the developing sleeve 1402, being a toner feed roller, and a developing blade 1401. The developing blade 1401 is usually one made of a metal or a resin. In the case of a resin blade being used, it is in contact with the developing sleeve 1402 under appropriate contact pressure. With rotation of the developing sleeve 1402 itself the toner layer formed on the developing sleeve 1402 comes to face to the photosensitive drum 11, and only the portions of V.sub.L are selectively developed by a voltage Vdc applied to the developing sleeve 1402 and the electric field formed by the surface potential of the photosensitive drum 11. The toner image on the photosensitive drum 11 is successively transferred onto a sheet fed from a sheet supplying device by a transfer unit 15. The transfer unit may be a corona charger as shown, or a unit of a transfer roller method in which the sheet is conveyed as applying a transfer charge to the sheet by supplying an electric current from a power supply to an electrically conductive, elastic roller. The sheet with the toner image transferred thereon is further fed to a fixing unit 10 with rotation of the photosensitive drum 11 to heat and press the toner image into a permanently fixed image.
The heat roller method as shown in FIG. 12 has widely been used heretofore for image heating apparatus typified by the heat fixing device.
The heat roller method, however, had the problem that the fixing roller has a large heat capacity to require high power for heating and a long wait time.
Thus, the following proposals have been made to directly heat the fixing roller by utilizing generation of induced current.
Japanese Utility Model Application Laid-open No. 51-109737 discloses the induction heat fixing device for inducing the electric current in the fixing roller by magnetic flux to heat it by Joule heat.
Further, Japanese Patent Publication No. 5-9027 discloses the heating technique utilizing the feature of the fixing roller being a rotating body, in such structure that an exciting coil is provided upstream of the nip in the rotating direction of the fixing roller.
In addition, U.S. Pat. No. 5,278,618 discloses an example using a fixing film of decreased heat capacity in place of the fixing roller and heating it by an exciting member near the nip.
The fixing device disclosed in Japanese Laid-open Utility Model Application No. 51-109737, however, had the drawback that radiation losses are large, because energy of alternating magnetic flux generated by the exciting coil is used for increasing the temperature of the entire fixing roller, and the density of the fixing energy is low with respect to the input energy so as to result in low efficiency.
Further, the fixing device disclosed in Japanese Patent Publication No. 5-9027 is arranged to use the magnetic flux energy in the local place, so that the radiation losses would be decreased. However, since it uses the magnetic flux penetrating the heated member, it is necessary to set a low frequency of the alternating current used for excitation, which caused the problem that the energy conversion efficiency is lowered.
U.S. Pat. No. 5,278,618 shows inclusion of the fixing film as a heated member in a part of the magnetic path, but it requires the magnetic path of very high flux density against the fixing film, because the magnetic path is limited in the nip area, which caused the problem of experiencing magnetic saturation and in turn failing to obtain sufficient efficiency.